1. Field of the Invention
The invention relates to methods and apparatus for erasing printed characters and more specifically to erasing by striking over the unwanted character.
2. Statement regarding the Prior Art
A great boon to typists, regardless of skill level, resulted when erasing shields and the unpleasant time consuming "rub-off" erasing procedure for typed characters gave way to "strike-over" erasing, i.e., erasing that involves a repeat striking of the unwanted character using either a cover-up material or an adhesive tape that lifts the unwanted character off the paper. Such strike-over erasing is faster and cleaner than the old procedure, and generally results in less noticeable erasures in the end product original.
Nevertheless, many sophisticated impact typing machines do not feature strike-over erasing. One possible reason for omission of this highly desirable feature is that the character placement accuracy of moderately-high-speed impact printers, such as the flexible-spoke ("daisy wheel") printing devices, is not sufficient to achieve an apparently complete erasure. More specifically, while type-bar and type-ball printers are usually adjusted to permit a return impact within the "lift-off zone" of a printed character--typically within .+-.0.005 cm of the original impression--medium to high speed printers (say those capable of printing at rates exceeding 30 characters per second) are generally incapable of consistently providing such strike-over accuracy. With flexible-spoke printers, this accuracy problem is thought to result mainly from vibrations in the long slender spokes and from striker alignment variations these being additive to the usual carriage positioning variations. Whatever the causes, it appears that significantly increased strike-over accuracy is possible only at an unduly high cost, if at all.
To avoid the above problems with overstriking characters for erasure purposes, recourse has been taken to erase techniques using block characters. These block characters have specially selected shapes that either singly or in cooperating groups cover the impression area for any given printed character. This special character overstriking technique requires, however, the addition of erase characters which are not useful for printing purposes and presents problems because the necessary striking force to make such characters effective for erasing, with their relatively large impact areas, results in paper blemishes that impair the appearance of the end product original.
Apparently because of the above outlined problems, medium to high speed printing units, as was mentioned above, typically do not feature strike-over erase and, if unwanted characters have been printed, they must either be erased with an abrasive eraser or painted over manually; otherwise, the page must be repeated to achieve a clean copy.